A receptor-like protein-tyrosine phosphatase PTP zeta/RPTP beta binds a heparin-binding growth factor midkine - Involvement of arginine 78 of midkinein the high affinity binding to PTP zeta

Midkine is a 13-kDa heparin-binding growth factor with 45% sequence identit y to pleiotrophin. Pleiotrophin has been demonstrated to bind to protein-ty rosine phosphatase zeta (PTP zeta) with high affinity. In this study, we ex amined the binding of midkine to PTP zeta by solid-phase binding assay. Mid kine and pleiotrophin binding to PTP zeta were equally inhibited by soluble pleiotrophin and also by some specific glycosaminoglycans. For both bindin gs, Scatchard analysis revealed low (3.0 nM) and high (0.58 nM) affinity bi nding sites. These results suggested that PTP zeta is a common receptor for midkine and pleiotrophin. Midkine is structurally divided into the N- and C-terminal halves, and the latter exhibited full activity for PTP zeta bind ing and neuronal migration induction. The C-terminal half contains two hepa rin-binding sites consisting of clusters of basic amino acids, Clusters I a nd II. A mutation at Arg(78) in Cluster I resulted in loss of the high affi nity binding and reduced neuronal migration-inducing activity, while mutati ons at Lys(83) and Lys(84) in Cluster II showed almost no effect on either activity. Chondroitinase ABC-treated PTP zeta exhibited similar low affinit y binding both to the native midkine and midkine mutants at Arg(78). These results suggested that Arg(78) in midkine plays an essential role in high a ffinity binding to PTP zeta by interacting with the chondroitin sulfate por tion of this receptor.